Wheel Speed Sensors

Anti-lock Braking System System

This section includes the ABS codes that are reported as part of the vehicle emissions certification.

Wheel Speed Sensors
The ABS modulates brake pressure on each wheel independently to maintain vehicle stability during braking.

The ABS continually monitors the rotational velocity of each wheel anytime the ignition switch is in the on position and determines if a tire is skidding when the brakes are applied. Only then does the ABS intervene to modulate the brake pressure to the skidding wheel. The modulation continues until the wheel rotates freely. The brake pressure is then restored and the modulate/restore cycle is repeated whenever skidding is detected. This cycle occurs at a rate of several times per second.

The ABS module is capable of detecting the following system conditions:
- Hydraulic valve failure.
- Wheel speed sensor failure.
- ABS power relay short circuit.
- Interconnect failures to the ABS sensors, power and ground to the ABS module.
- Over/under voltage conditions.

The ABS provides failure messages, via the ABS indicator, in the ipk. Failure of the ABS module, for whatever reason, will not compromise the normal operation of the brake system.

Wheel Speed Sensor Monitoring (XJ Range, XK Range and S-Type)
The ABS system monitors all four wheel speed censors continuously. A number of checks are performed, the failure of any one will cause the ABS system to be disabled and the ABS warning lamp together with the MIL to be illuminated. The monitors are performed differently on the X-Type and, therefore, the description for this system is dealt with separately.

Sensor Signal Current Out of Range
The current from each sensor is continually monitored against an upper and lower threshold. If the current is outside the threshold limits a counter is incremented and the check re-run. When the counter reaches its predefined limit the DTC for the appropriate wheel speed sensor is logged.

Missing Wheel Speed Sensor Input
If any of the wheel speed signals is lost (assuming normal signals from the other three wheels) for more than 0.007 seconds, the DTC for the appropriate wheel speed sensor is logged.

Wheel Speed Sensor Signal Continuously too Low
If the signal current from any wheel speed sensor is identified as being too low for more than a defined period then the DTC for the appropriate wheel speed sensor is logged.

Comparison of Maximum Wheel Speed Versus Minimum Wheel Speed
This monitor compares the difference in wheel speed of the sensors over a long period (180 seconds). If the difference between the maximum and minimum wheel speeds continuously exceeds the defined threshold the DTC for the appropriate wheel speed sensor is logged.

Wheel Speed Signal Changes Erratically
Erratic wheel speed signals are monitored by checking the variation in successive samples. If the difference in signal from each successive sample is greater than 15.5 mph or the signal interrupt is detected (no sample) then a software counter is incremented. If the counter reaches its defined limit the DTC for the appropriate wheel speed sensor is logged.

Periodic Drops of Wheel Speed Signal
At wheel speeds above 12.4 mph, each sensor is monitored for loss of wheel speed signal. If the signal is lost for more than 15 revolutions then the DTC is logged.







Wheel Speed Sensor Monitoring (X-Type)

Dynamic Monitoring
The monitor runs when the vehicle speed reaches 24.8 mph. If any of the wheel speed signals is lost (assuming normal signals for the other 3 wheels) for more than 20 ms the DTC for the appropriate wheel speed sensor is logged.

Static Start-up Monitoring
This monitor checks for the loss of the wheel speed signal at ignition on. If any wheel speed signal is not present for more than 20 seconds then the appropriate DTC is logged.

Drive-off High Speed
This monitor checks for loss of a wheel speed sensor signal during rapid acceleration from being stationary. The monitor looks for one wheel speed signal being stuck at 0 mph when the other three are greater than 11 mph. If this situation occurs, a timer is started. If after 0.020 seconds the situation still exists then the DTC for the appropriate wheel speed sensor is logged.

Drive-off Low Speed
This monitor checks for the loss of a wheel speed sensor signal during slow acceleration from being stationary and during continued low speed driving. If the difference between the maximum and minimum wheel speed continuously exceeds any of the defined thresholds for more than 20 seconds the DTC for the appropriate wheel speed sensor is logged.

Static Wheel Slip
This monitor compares the difference in the wheel speed of the sensors over a longer period of time (5 seconds) during normal driving. If the vehicle speed is below 62 mph, then the wheel speed sensors are checked for either the deviation of the two wheel speeds at either side of the vehicle being greater than 3.7 mph, or the deviation of the wheel speed at the front axle being greater than 6.2 mph. If at least one wheel is at 3 mph or lower, a wheel speed deviation of adjoining wheel of 7.4 mph is permitted. If the detected deviation exists for more than 5 seconds then the appropriate DTC is logged. If the vehicle speed is greater than 62 mph then the wheel speed sensors are checked for either the deviation of two wheels speeds at either side of the vehicle being greater than 6% or the deviation of wheel speeds at the front axle being greater then 2.5 mph +6%. If the detected deviation exists for more than 5 seconds then the appropriate DTC is logged.

Ohmic Monitoring
This monitor performs a static impedance check on each wheel speed sensor when the ignition is switched on. If the impedance of any sensor is outside of its defined limits, then the appropriate DTC is logged.